Polypropylene films are common packaging materials because of their relative low cost and moisture resistance. However, polypropylene has a fairly high permeability to gases, including oxygen. Therefore, films composed entirely of polypropylene are not adequate for packaging oxygen sensitive foods and other materials that degrade in the presence of oxygen or other atmospheric gases.
Schroeder et al, U.S. Pat. No. 4,254,169, the entire disclosure of which is expressly incorporated herein by reference, describes the use of an ethylene vinyl alcohol layer (EVAL) as an oxygen barrier layer. The EVAL layer sandwiched between two layers of polyolefin in a coextruded multi-layer film. The multi-layer film of the Schroeder, et al patent is not biaxially oriented.
Odorzynski et al, U.S. Pat. No. 4,347,332, discloses films with EVAL barrier layers which are modified with additives such as toluenesulfonamides. U.S. Pat. No. 4,347,332 is incorporated herein by reference.
In order to achieve optimal oxygen impermeability in a multi-layer film such as described in the above-mentioned Schroeder patent, it is necessary to use an ethylene vinyl alcohol copolymer having a relatively high vinyl alcohol to ethylene monomeric ratio. But such films have extremely poor rheology characteristics compared with polypropylene films. Consequently, composite films containing polypropylene and ethylene vinyl alcohol copolymer are not readily biaxially orientable.